Traditionally, sand molds have been used for casting in the foundry and ceramics industries. The sand molds are typically a mixture of sand and binders (or sand otherwise chemically treated) which is tailored to form a mold and core for the particular item to be cast. Once the item has been cast, the mold and core must be broken to remove the casting.
Instead of discarding the broken molds, it has been common practice in the industry to recycle the molds to reclaim the sand for reuse in subsequent molds. This generally includes both physical and thermal reclamation. First, the lumps comprising the broken molds must be reduced to return the sand to usable granular form and to separate any tramp metal or oversized material from the reclaimed sand. It will be appreciated that broken sand molds are extremely hard and thus difficult to break up such that special equipment has been developed for this purpose. For example, U.S. Pat. No. 4,050,635 to Whirl-Air-Flow Corporation, the assignee of the present invention, shows a sand reclamation apparatus which is well suited for this purpose.
After physical reclamation, the reclaimed granular sand must then be treated to remove residual coatings for reuse as new sand supplement. This additional treatment is usually performed by heating the sand to an elevated temperature. Such thermal reconditioning usually takes place in a calciner which is, in effect, a special kiln in which all hydrocarbons and any remaining resins or binders are cleaned from the granular sand. Thermal treatment also stabilizes the sand expansion characteristics, a very desireable feature.
Various types of sand calciners or thermal reclamation systems have been available heretofore, however the devices of the prior art have tended to be bulky, slow, inefficient, overly expensive, or otherwise unsatisfactory for one reason or another. For example, the old Nickles-Hershoff machine utilized a multiple hearth tower which was fired tangentially and which incorporated a center shaft with plows for plowing the sand back and forth between levels of fire brick. This machine, however, is expensive and difficult to maintain, and difficult to control during operation. The Coreco indirect fired kiln from College Research Corporation of Germantown, Wis., incorporates an inner tube which is heated by burners extending through a surrounding refractory lining such that there is no direct flame impingement on the sand to assure thorough calcining and complete removal of the binders. Finally, the SAND SOURCE thermal reclamation system from Combustion Engineering, Inc. of Pittsburgh, Pa., incorporates a rotary drum with a burner extending into one end which does not permit properly controlled temperature distribution within the drum.
There is thus a need for a new and improved method and apparatus for calcining sand in which fluidized sand is indexed through a rocking or oscillating drum in a manner which causes thorough mixing and exposure of the sand to multiple burners located along the top side of the drum such that more effective calcination is achieved.